Electrolytic production of pinacols



June 17, 1947. 11s, c s r 2,422,468

ELECTROLY'IIIC PRODUCTION OF PINACOLS Filed July 4, 1942 CAT Hooze a W a COOLING .34 I/ Til/M LEA-17 COAT/NC Patented June 17, 1947 Thomas S. Chambers, Newark, and Ober C.

Slotterbeck, Rahway, N. J assignors to Standard Oil Development Company, a corporation of Delaware Application July 4, 1942, Serial No. 449,718

The present invention pertains to a method of preparing vicinal glycols by electrolytic reduction of ketones and generally to all types of carbon to carbon reductive condensation reactions.

Specifically, our invention is concerned with th reduction of acetone electrolytically to tetramethylethylene glycol.

It is the object of the present invention to provide the art with a novel and high advantageous method of electrolytically forming vicinal glycols and especially pinacols from the corresponding ketones, and also to provide the art with novel electrodes for effecting the reduction of organic compounds.

The electrolytic reduction oi. ketones can take place at the cathode surface according to any one of the following reactions:

Other side reactions result in the formation. of metal alkyls, especially when the reduction is eiiected in acid medium with lead cathodes.

An electrolytic method for effecting the reductive condensation reaction whereby acetone is converted to pinacol (tetramethylethylene glycol) using a lead cathode and an acid catholyte is described in German Patent 113,719 (1899). The claims of the Germany patent were verified by Elbs in Z. Electrochemie, 7,644 (1901).

Several German patents pertaining to the electrolytic reduction of acetone to pinacol were issued in the period 1912-1920. Among these patents were DRP. 306,304 (1917), 306,523 (1918) and 324,919 (1920) The principal developments made in this period appear to have been the use of mixed metal and alloy cath'odes, such as 4 to copper-96 to 90% lead mixtures and 10% tin-90% lead alloy, and the use of horizontally suspended cathodes to prevent the metal alkyls formed from accumulating on and decreasing the activity of the electrode surface.

All of the prior processes for electrolytically converting acetone to pinacol have been unsatisfactory either because pinacol current efliciency or the overall current efficiency or both were too 6 Claims. ((1204-77) low. The current emciency (C. E.) is the ratio of the theoretical quantity of electricity required for a given reduction to that actually used. Thus, the pinacol C. E.

grams of pinacol hydrateX 53.6X 100 mol. wt. of pinacol hydrateXtotal ampere hours reduction of ketones to vici'nal glycols effected with good current efliciency and high ratio of glycols to other reduction products by utilizing as the cathode a low hydrogen overvoltage metal having a thin film of lead deposited thereon.

' The ketones which may be treated in accordance with the present invention correspond to the general formula CH5 CH3 or methyl ethyl ketone may be condensed for example with acetophenone to form the glycol The glycols formed in accordance with the present invention correspond to the general formula:

panying drawing.

Figure 1 is a plan view of a single unit cell and Figure 2 is a vertical cross-section of a multiple unit cell.

Figure 3 is an isometric vie w of the cathode of the present invention.

The single unit cell consists of a rectangular jar or the like b, arranged in a cooling bath a. An anode e of lead or the like is arranged inside an acid resistant diaphragm d, which divides the cell into an anode and cathode compartment. As shown, two cathodes c of copper or other metal of low hydrogen overvoltage having a thin coating of lead are arranged in the cell.

In the multiple unit cell shown in Figure 2, l is a rectangular acid resistant tank; 2 are diaphragms which may be made of suitable acid resistant material; .3, are the lead coated cathodes, while 4 are the anodes which may be made of lead, carbon, or the like. 5 is a pump for cir- 'culating the catholyte; 6 are cooling coils for controlling the temperature of the electrolyte. The cooling fluid passing through coils 6 is circulated through line I, cooling bath 8 and lines 9 by circulating pump Ill. The details of the cell are not critical to our invention and the cell may be altered in numerous ways. For example, the process could be carried out continuously by providing a circulating pump, an overflow means for drawing oil the catholyte from the cell, means for separating electrolysis products from the withdrawn catholyte and means for making the catholyte up to initial strength for reintroduction into the cell.

Figure 3 illustrating the electrode in accordance with this invention has part of the lead coating removed to show the base.

The use of a diaphragm is recommended since the products formed at the cathode are liable to be oxidized at the anode. The use of a diaphragm may be avoided if desired, however, by the use of anolyte and catholyte of unequal densities in cells provided with horizontal electrodes, by the use of a high anodic current density or by the use of anodes having a low oxygen overvoltage, 1. e., nickel or by'a "combination of two or more of these expedients.

The metals of low hydrogen overvoltage that may be used as the base for our cathode include copper, nickel, silver, gold and platinum or alloys such as Monel metal and brass.

The film of lead is preferably electrodeposited on the low overvoltage metal base in the following manner: Plates of the metal which are to serve as the base of the cathode are treated for a few minutes with one part of nitric acid to three parts of distilled water. The plates are washed with water to remove nitric acid and are then ready for lead plating. The 'platingbath contains about 50 to 100 grams of lead nitrate per liter of distilled water. Lead sheets are used as the other electrodes and upon assembly of the cell, the plates which are to receive the lead plating are made the anodes and the lead sheet is made thecathode at a current density of about 1.55 amperes/sq, dm. for minute. The current flow is then reversed so that the plates are cathodes and the lead sheets the anodes. Plating is continued for about 2 minutes at the same or a higher current density. The lead coated plates are then removed from the plating bath and immediately washed with .hot water and dried, whereupon they may be used directly without any further conditioning'treatment. It is to be understood that this method of preparing our lead coated cathodes is merely illustrative and that other methods can be utilized. For example, the lead can be deposited on the base plates from fiuosilicate, fiuoborate and perchlorate baths. The important factor in the preparation 0f the cathode is that the lead deposit must be kept very thin, preferably within the range from about 0.02 to about 0.5 gram/sq. dm.

Aside from the nature of the cathode surface, the factor which has the greatest effect upon the course of the reaction in the cell is the concentration and type of electrolyte used. Acid media have given the best results. The ratio of ketone to water in the catholyte should be high. However, a ratio of 4/1 appears to be approximately the upper limit economically because of the high It is noted that where reference to strength of sulfuric acid is made, it should be understood that the strength indicated is weight percentage.

The anolyte may be a sulfuric acid solution of about 15. to 40% strength. The electrolyte is preferably maintained at temperatures between about 0 C. and about 25 C. The current density applied may vary between about 0.1 and about 4 amperes/sq. dm., the preferred current density being within the range of about 1 to about 2 amperes/sq. dm.

The following examples serve to illustrate our invention but it is to be understood that our invention is not limited thereto.

Example 1 A single unit cell as shown in Figure 1 having a capacity of about 4 liters was provided with a lead anode. The cathodes were copper sheets about 21 x 20 cms. and about 0.2 cm. in thickness. The copper sheets were treated as described above and electroplated in a solution of lead nitrate containing 100 grams of lead nitrate/liter at 20 amperes for 2 to 3 minutes. The catholyte consisted of a 4/1 acetone Iii-20% sulfuric acid solution. The anolyte was 20% sulfuric acid. The electrolysis was conductedat 010 C. at a current density of 1.55 to 2 amperes/sq. dm. The run was conducted for 21 hours.

Upon conclusion of the electrolysis, isopropyl alcohol and unreacted acetone were stripped from the catholyte under reduced pressure at low temperatures (below 25 C.). methylethylene glycol) was separated from the residue as the hexahydrate by cooling and filtering the crystals.

656 grams of pinacol hydrate corresponding to 343 grams of tetramethylethylene glycol and 14 grams of isopropyl alcohol were obtained. This correspond to an overall current. efllciency of 44% based on pinacol and 3.6% based on isopropyl alcohol. The mol ratio of pinacol hydrate to isopropyl alcohol was 12.5/1.

When using 4% copper-96% lead, or 10% copper-% lead mixtures in sheet form as the cathode in lieu of the lead deposited on copper cath-' odes with 25% sulfuric acid anolyte and 4/1 ace-' tone 25% sulfuric acid catholyte, electrolyzed at 16 C. at a current density of 1.67 amperes/sq. dm. for 6 and hours respectively, current emciencies based on pinacol of 12.7% and 13.8%, respectively,

and a mo] ratio of pinacoi hydrate/isopropyl alcohol of0.4/1 and 0.9/1, respectively were obtained.

Example 2 A multiple unit cell as shown in Figure 2 was a recycle receptacle over each 24 hour period.-

The recycle receptacle was maintained at a temperature about 10 to 20 C. below that of the cell, thereby continuously removing some pinacol hydrate (tetramethylethylene glycol hexahydrate) from the catholyte.

The data from the run is summarized in the following table:

phragm cell in contact with a cathode comprising a sheet of copper, completely covered with a thin, electrodeposited layer of lead weighing from about 0.02 to about 0.5 gram/sq. am.

2.11: the process of producing 81Ycols correspending to the general formula v 4 FR n 11 wherein R stands for a member of the group consisting of alkyl and aryl radicals and substituted alkyl and aryl radicals by the electrolytic reduction of an aqueous sulfuric acid solution of a ketone of the formula wherein R stands for a member of the group consisting ofalkyl and aryl radicals and alkyl and aryl radicals substituted by a group which is not reducible under. the reaction conditions employed, the improvement which comprises effecting said reduction at a temperature below about 25 C. and at a current density of between about 0.1 and about 4 amps./sq. dm. in a diaphragm cell in contact with a cathode comprising a sheet of copper, completely covered with Tetramethyl ethylene Current Efliclen Cathode Ampere glycolliexahydratc-g. figaggf' Based on Pinacgf Mol Ratio Hour 011115386 Total Pinacollllsim Total Interval Total Interval Total Interval pmpy 112 112 240 240 IIIIIIIIII 52.1, 352 250 100 454 45.0 544 152 150 35.2 325 as s5 as 183 51.2 181 to are 195 49. 5' 435 304 ass 310 50.0 33.0

110 no 254 .254 545 205 as s44 00 40.3 22. 755 540 464 120 20.3

I Readings and analyses were made at interval times stated.

What we claim and desire to secure by Letters Patent is: 1

1. In the process of producing glycols corresponding. to the general formula wherein R stands for a member of the group consisting of alkyl and aryl radicals and alkyl and aryl radicals substituted by a group which is not reducible under the reaction conditions employed, the improvement which comprises effecting said reduction at a temperature below about 25 C. and at a current density of between about 0.1 and about 4 amps/sq. dm. in a diaa thin, electrodeposited layer of lead weighing from about 0.02 to about 0.5 gram/sq. dm.

. 3. The process as defined-in claim 2 wherein the ketone and the sulfuric acid are used in the ratio of 4 volumes of ketone to one volume of acid.'

4. The process of producing tetramethyl ethylene glycol by the electrolytic reduction of an aqueous acid solution of acetone, the improvement which comprises effecting said reduction at a temperature below about 25 C. at a current density of between 0.1 and about 4 amps/sq. dm. in a diaphragm cell in contact with a cathode comprising a'sheet of copper, completely covered with a thin, electrodeposited layer of lead weighing from about 0.02 to about 0.5 gram/sq. dm.

5. The process of producing tetramethyl ethyl-- ene glycol by the electrolytic reduction of an aqueous sulfuric acid solution of acetone, the improvement which comprises effecting said reduction at a temperature below about 25 C. at a current density of between 0.1 and about 4 amps./sq. dm. in a diaphragm cell in contact with a cathode comprising a sheet of copper, completely covered with a thin, electrodeposited layer of lead weighing froin about 0.02 to about 0.5 gram/sq.

6 The process as defined in claim 5 wherein.

the ketone and the sulfuric acid-are used in the ratio of 4 ,volumes of ketone to one volume of acid. I

'I'I-IOMAS S. CHAMBERS. OBER C. SLOITERBECK.

REFERENCES CITED 1 The following references are of record in the file of this patent:

, UNITED J STATES PATENTS Number l FOREIGNIA'I'ENTS' Number Country Date 252,759 Germany Oct; 26, 1912 306,523 Germany- June 26,1918 306,304 Germany Dec. 10, 1919 310,023 Germany Dec; 29, 1920 113,719 Germany Sept. 7, 1900 2 OTHER.- REFERENCES Transactions of the AmericanElectrochemical Society, vol. 27, 1915',--pages 131+140;*vol. 80, 1941, pages 139-150. (Copy inzllivision 56 of the Patent'Oiflce Tainton et al article in g 'l'ransa ctions of the 15 American Institute oi, Mining-and Metallurgical Engineering, vol. 70 (1924),;pages516, 518, 519. I

(Copy in Division 3.)" 

